The invention relates to an audio system comprising a circuit for processing an audio signal, whereby the circuit comprises an input for receiving the audio signal and an output for supplying an output signal, a harmonics generator coupled to the input for generating harmonics of the audio signal, adding means coupled to the input as well as to the harmonics generator for supplying a sum of the audio signal and the generated harmonics to the output.
The invention further relates to a circuit for processing an audio signal, a harmonics generator and a method for processing an audio signal.
An audio system according to the preamble is known from EP-A 546 619. Since the invention of the electrodynamic loudspeaker, there has been a need for greater acoustical output, especially at low frequencies. Often however, for instance in television sets or portable audio sets, this acoustical output is severely limited by the small size of the loudspeakers. It is known that this dilemma can be solved by using a psychoacoustic phenomenon often referred to as virtual pitch or missing fundamental, which evokes the illusion of a higher bass-response, while the loudspeaker does not radiate more power at those low frequencies. This illusion can be created by replacing low-frequency tones, which are present in the audio signal but cannot be reproduced by a small loudspeaker, by harmonics of these tones. The harmonics now represent the low-frequency tones.
In the known audio system a low-frequency band of an audio signal is selected and supplied to a harmonics generator for generating harmonics of the selected signal. The generated harmonics are thereafter added to the audio signal. In this way the low-frequency perception of the audio signal is improved. In the known audio system a full-wave rectifier is used as the harmonics generator, which generates only even harmonics. Some low-frequency tones, which are reproduced by the known audio system, are perceived by human beings as having a higher loudness than the loudness of the corresponding low-frequency tones which are present in the audio signal.
An object of the invention is to provide an audio system, wherein the perceived loudness of low-frequency tones is substantially equal to the received loudness of the corresponding low-frequency tones in the original audio signal. This object is achieved in the audio system according to the invention, which is characterized in that the harmonics generator is embodied so as to limit the amplitude of the generated harmonics. Experiments have shown that, by this measure, the perceived loudness of low-frequency tones can be controlled accurately, thus allowing this perceived loudness to be substantially equal to the received loudness of the corresponding low-frequency tones in the original audio signal.
An embodiment of the audio system according to the invention is characterized in that the harmonics generator is embodied so as to fix the amplitude of the generated harmonics when the amplitude crosses a threshold value. In this way, this embodiment incorporates a simple and advantageous realisation of the desired limitation of the amplitude of the generated harmonics.
A further embodiment of the audio system according to the invention is characterized in that the harmonics generator comprises a rectifier for rectifying the audio signal. This measure enables the harmonics to be generated in a simple and effective way.